The general objectives of the proposed research are to understand mechanisms of bacteriophage assembly and conformational change, using quasi-elastic laser light scattering and other biophysical techniques, such as electron microscopy, sedimentation equilibrium, sedimentation velocity, circular dichroism, and difference spectroscopy. In particular, we hope to (1) determine the mechanism which regulates the length of the tail core of bacteriophage T4; (2) determine the conformational and energetic factors that regulate whether core protein remains monomeric or forms helical polymers; (3) determine the factors which regulate the polymerization of sheath protein and its interaction with the core; (4) determine the chemical factor responsible for head-tail joining in T4; (5) measure the rate of tail fiber attachment to fiberless phage particles, and characterize the catalytic action of phage gene product P63 in this regard; (6) understand the mechanism of the tail fiber transition, regarded as an allosteric transition, with respect to the action of ions and cofactors and the difference in cooperativity between T2 and T4 phages; and (7) use proflavin binding to characterize the state of intraphage DNA and the permeability of the protein capsid in relation to packing density, osmotic shock resistance, and interactions with polyamines; and to examine psi-DNA as a model for intraphage DNA.